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With three mics you can go for the classic Glyn Johns technique with one mic front of kick one three stick lengths above the snare and one low pointing across the floor tom three stick lengths from the centre of the snare too! If you can try this with a great kit in a great room with a great player and three U67s in great condition, you're gonna be VERY VERY happy!!! Pan the two kit mics L and R and leave the kick in the middle! Obviously thats an ideal setup so substitute away and see what happens!
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You shouldn't have too much problem with the drums leaking onto the other mics apart from the vocal mic. Place the guitar mic right up to one of the speakers in the speaker cab, try to use a DI box for the bass - it will sound better anyway. I am assuming your band is made up of the usual instrumentation and does not incorporate a grand piano or violin!!! As for the kit - well with only 3 mics I would place one on the kick, one on the snare and lose the 3rd - if you are just monitoring live. For recording then still keep one on the kick, then use the other 2 as a stereo pair a la Glyn Johns mode - one between the rack tom and hi hat facing the snare and one above the floor tom also facing the snare and both equidistant from the snare. Pan the channels left and right. You will need to experiment with the positioning depending upon the set-up of your kit but you could get some great results.